Several different types of electrically controllable active filters are well known, as well as their design and manufacture in the form of integrated circuits. It is further known to tune active filters with the aid of an outside phase-locked loop which is locked to a given frequency. A general description of the design and control of active electrical filters for realization in the form of integrated circuits is presented in the article "Analog integrated filters or continous-time filters for LSI and VLSI", REVUE DE PHYSIQUE APPLIQUEE, January 1987, No. 1, pp 3-14. Further information as to the design and control of active filters is given in the references found on pages 13 and 14 in the article.
The U.S. Pat. Nos. 4,288,754 and 4,306,198 teach the use of differential amplifiers with groups of series-connected diodes in the design of electrically controllable active filters.
From the U.S. Pat. No. 4,379,268 it is known in other applications than filters also to use differential amplifiers with groups of series-connected diodes.